Conventional down-leads have been known for some time now, and a variety of papers and articles detail their design, including: Yu. L. Buyanov, et al., Cryogenics, 15 p. 193-200 (1975); Yu. L. Buyanov, Cryogenics, 25 p. 94-110, (1985); H. L. Nan, Cryogenics, 23, p. 501-507 (1983); and, J. J. Alff, Proc. 9th International Conf. on Magnet Technology, September 1985.
There are also many articles relating to the design of high temperature superconducting current down-leads, among them: F. J. Mumford, Cryogenics29 p. 206-207 (1989); J. R. Hull, IEEE Transactions Appl. Supercond., 3 p. 869-875 (1993); R. Wesche and A. M. Fuchs, Cryogenics, (1994); A. Matrone, et al., IEEE Transactions Magn., 25 p. 742-1745, (1989); and, M. A. Green, Cryogenics, 30S p. 679-683 (1990).
Very low temperatures are necessary to enable superconducting material to exhibit its properties. However, power must be supplied to these superconducting devices operating at cryogenic temperatures and most power sources are often at room temperature, or about 300 K. In order to drop the temperature of the power conductors and/or connections to the operating range of the superconductor material (which is about 4 k) and then maintain that temperature, supercooled liquids are often used. This is because the large currents that the conductors carry generate heat due to their resistive properties. Superconductor leads must isolate the heat generated by these large currents with thermal insulators, but instabilities and heat leaks from the material can still occur.
Heat leaks also occur in superconductors as a result of connections between the low temperature superconductor (LTS) and the high temperature superconductor (HTS). Even small thermally conductive areas in the electrical leads can cause large heat leaks into the supercooled regions that will disrupt the operation of the superconductor.
In U.S. Patent 5,324,891 to Huang et al., a superconducting lead with a thermal plug is disclosed. The invention therein consists of multiple strands of superconducting material in contact with a stainless steel and copper alloy laminae along the length of the strands. This copper/stainless steel laminae acts as a safety lead whose purpose is to add thermal mass in order to help prevent the lead from losing its superconductive properties. The ends of the strands contact conductors that are at different temperatures. Liquid nitrogen is used to cool the warm end of the lead and liquid helium cools the cold end. This lead allows a room temperature power source to be connected to a superconducting material.
Problems with this type of lead are found in its ability to handle current variances and its lack of an electrical safety lead. Also, if one of the HTS tapes used in the lead fails, it is difficult, if not impossible, to remove and replace it. Furthermore, there are no provisions for the safety lead of this device to also carry current, it is designed solely to provide additional thermal mass.
It is thus an object of this invention to provide a superconductor lead that has a safety lead incorporated therein that does not adversely affect the superconducting properties of the HTS material used in the lead. Another object of the present invention is to provide a lead wherein the components of the lead are modular and may be attached and detached from one another with relative ease. A further object of the invention is to provide for greater strain relief on the HTS elements as they are cooled to operating temperatures. Another object of the invention is to incorporate a safety lead that provides both thermal mass and an electrical bypass to the HTS material. Still another object of the invention is to provide independent, and improved, gas cooling means to the modular segments. Yet another object of the invention is to provide a means of removing and/or rotating the individual HTS elements depending on the magnetic characteristics desired.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific objects attained by its uses, reference is made to the accompanying drawings and descriptive matter in which a preferred embodiment of the invention is illustrated.